Methods utilizing the Doppler phenomenon for measuring the velocity of one body with respect to another are known in the art. Many methods are directed towards vehicle applications. In such applications, an apparatus is typically mounted at the rear or on the underside of a vehicle so as to transmit wave energy to an underlying surface. The apparatus receives wave energy reflected from the surface and calculates the Doppler shift to determine the speed or distance travelled by the vehicle with respect to the surface.
In theory this method for determining velocity works quite well. In practice, however, prior art methods prove adequate at best, and unsatisfactory in harsh or rapidly changing environments. Transmitter and receiver placement shown in the prior art promotes inherent problems in a velocity measuring system of this type. Some apparatus use a single transmitter/receiver mounted on the rear of a vehicle. Changes of the slope of the surface over which the vehicle travels introduce Doppler shifts which render false velocity information. The "pitching" of land vehicles as they travel over uneven terrain causes the sensor to move relative to the surface, thus inducing Doppler shifts which are not wholly indicative of vehicle speed but vehicle pitch as well. Similar devices used on ships deliver inaccurate information due to irregularities on the sea floor.
Other transmitter/receiver placements attempt to overcome inaccuracies found in these single-sensor devices. U.S. Pat. No. 4,506,353 issued to Rott et al on Mar. 19, 1985; U.S. Pat. No. 3,745,520 issued to Barret et al on July 10, 1973; and U.S. Pat. No. 3,893,076 issued to Clifford on July 1, 1975 disclose vehicles having two transmitter/receivers being centrally located on the underside of a vehicle, and positioned to transmit away from one another, one forwardly and one rearwardly. The Doppler shifts of each are compared one with another in order to eliminate a portion of the inaccuracies encountered due to a changing ground profile. These arrangements effectively eliminate the majority of vertical irregularities, such as vehicle bounce. However, since one transmitter/receiver is aimed away from the other, variations occur in this distance which effect the accuracy of the measurements. For instance, should the vehicle crest a hill, both transmitter/receivers will sense an increase in speed, while in fact a portion of the sensed speed is related to the pitching of the vehicle.
Additionally, cross talk between one transmitter/receiver and another causes apparent speed variations. Typically, both transmitters emit the same frequency. When the reflected waves impinge on the receivers, the receivers cannot distinguish between the signals sent from their transmitter and the other transmitter. This situation usually causes inaccuracies in calculated speed, or signal loss due to destructive interference. Moreover, the demodulation schemes used by a variety of known Doppler speed detectors are largely analog. Phase locked loops attempt to lock on to the incoming reflected signal and accurately determine its frequency. However, multiple out-of-phase reflections are difficult to lock on, and commonly produce errors in detected frequency. If the frequency of the received signal cannot be accurately determined under severe conditions, such as vehicle applications, the device cannot render an accurate indication of velocity.
Other environmental conditions besides a changing surface and vehicle pitch impose errors on many known detectors. Wind velocity, vehicle noise and vibration, and ambient temperature introduce inaccuracies into the detection schemes. Wind causes amplitude changes which are usually detected as signal shift, thus "fooling" analog detection schemes. Some sensors exhibit sensitivity to vehicle vibration, which corrupts the received signal and results in signal loss or poor velocity measurements. Changes in ambient temperature affect wave velocity and Doppler shift. Errors easily reaching 20% or more are encountered in some environments.